1. Field of the Invention
This invention relates to paper transport devices of the type utilized in connection with typewriters and printers having powered platens.
2. Prior Art
Transport devices for use in connection with margin perforated data carries, such as paper, wherein the transport is attachable to, and detachable from, the printer or typewriter are known.
In order to provide controlled movement of endless paper webs which are provided with left and right hand margin perforations and which are used in connection with printers, such as typewriters and the like, it has been known to provide paper transport devices adjacent the platen. The transport device may be coupled to the drive of the platen and may include toothed members engaging in the paper perforations. The toothed members, which may be sprockets or belts having projecting teeth, are commonly referred to as tractors or tractor drives. It is generally desirable to maintain a bias between the tractor drive and the platen.
Such bias, which is necessary for reliable and distortion-free transportation of the paper, is generally provided by stretching the installed paper web across a clamp or clamp roller positioned over the paper web. Such clamps are not always effective and can smear the print. It is also possible to produce the desired bias by displacing the tractors relative to the platen by means of mechanical devices. However, the prior bias creating methods require special measures and devices to produce the desired bias (stretch load) on the paper. It would therefore be an advance in the art to provide an improved attachable and detachable paper transport mechanism for use in connection with platen printing or typing devices which assures slip-free transportation of multi-layer marginal perforated paper webs, both forward and backward, by maintaining an effective bias on the web between the platen and the tractor mechanism.